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Stitch length: Stitch may be of longer or shorter.
The higher the stitch length the more the strength of stitch.

SPI: SPI means Stitch per Inch. It refers to stitch
density.

a. High SPI: Thread density is high as results inter cohesional
force between thread is increased. So, the strength of stitch is increased. But
for this requires more thread consumption and more time. Another problem may
arise is seam puckering due to high SPI.

b.Low SPI: Low SPI means low thread density. As a result
inter cohesional force between thread is comparatively less. So, strength of
stitch is comparatively low. The advantage of low SPI is less thread
consumption, less time required.

Thread tension: When stitch is formed there will be
two tensions is generated. i.e.: